In the design of piping systems or equipment, the primary goal is to ensure the system's integrity under applied loads. Typically, pressure design is based on preventing failure under steady loads, guided by failure theories. However, systems may fail even under safe load limits when subjected to specific types of cyclic loads. This article discusses the two broad classifications of loads, their failure modes, and design considerations.
Types of Failure
Catastrophic Failure:
- Occurs suddenly when a load far exceeds the design limit.
- Example: A pipe designed for a specific pressure ruptures if subjected to a much higher pressure, even once.
- This type of failure happens when the material's strength is overwhelmed by excessive stress.
Fatigue Failure:
- Occurs over time due to cyclic application of loads, even if the loads remain within safe limits.
- Example: A pipe experiencing repetitive cycles of high and low pressure may fail after prolonged use.
- Fatigue failure is not prevented by conventional design methods and requires special considerations.
To ensure system reliability, both types of failure must be accounted for during design.
Classification of Loads
The loads acting on a piping system or structural component are broadly categorized as:
Primary Loads:
- Definition: Loads caused by forces acting directly on the system.
- Examples:
- Internal or external fluid pressure.
- Weight of the pipe and its contents.
- Forces from pressure surges like water hammer or blowdown events.
- Characteristics:
- These loads are not self-limiting—stress and deformation persist as long as the load exists.
- Excessive primary loads can lead to plastic deformation and eventual rupture.
- Failure Mode: Catastrophic failure, which occurs when the material's yield or ultimate strength is exceeded.
- Design Approach:
- Based on failure theories like maximum stress or strain criteria.
- Ensures that the system can sustain primary loads without permanent damage.
Secondary Loads:
- Definition: Loads resulting from displacement or deformation, rather than direct forces.
- Examples:
- Tank settlement causing pipe displacement.
- Thermal expansion or contraction of pipes due to temperature changes.
- Vibration induced by connected equipment.
- Characteristics:
- Often cyclic in nature, such as thermal expansion during operation cycles.
- Some may be non-cyclic, like displacement due to permanent tank settlement.
- Failure Mode: Fatigue failure, caused by repeated cycles of loading and unloading, leading to progressive material degradation.
- Design Approach:
- Requires stress analysis and fatigue evaluation to account for incremental damage caused by cyclic loads.
Failure Mechanisms
Catastrophic Failure:
- Occurs at the microscopic level where individual grains or crystals of the material fail due to excessive stress.
Fatigue Failure:
- Happens at a mesoscopic or macroscopic level due to collective degradation of material properties over time.
- Incremental damage accumulates with each cycle, eventually leading to system failure.
Design Implications
- Primary Load Design:
- Focused on preventing catastrophic failure using traditional design methods and failure theories.
- Secondary Load Design:
- Requires additional considerations for cyclic loads, including thermal effects, displacement stresses, and fatigue resistance.
By addressing both primary and secondary loads, piping systems can be designed to withstand both immediate and long-term stresses, ensuring reliability and safety throughout their operational life.